drivers: i2c: nrfx_twim: avoid init/deinit to update frequency

nrfx HAL does not support updating frequency at runtime, which means we
need to do a full init/deinit to update it when it is a matter of a
simple register write. Fix this by using the lower level HAL.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>